The White House has confirmed that Ukraine is using US cluster bombs against Russian forces in the country.
National Security Spokesman John Kirby said initial feedback suggested they were being used "effectively" on Russian defensive positions and operations.
Cluster bombs scatter multiple bomblets and are banned by more than 100 states due to their threat to civilians.
The US agreed to supply them to boost Ukrainian ammunition supplies.
Ukraine has promised the bombs will only be used to dislodge concentrations of Russian enemy soldiers.
"They are using them appropriately," Mr Kirby said. "They're using them effectively and they are actually having an impact on Russia's defensive formations and Russia's defensive manoeuvring. I think I can leave it at that."
The US decided to send cluster bombs after Ukraine warned that it was running out of ammunition during its summer counter-offensive, which has been slower and more costly than many had hoped.
President Joe Biden called the decision "very difficult", while its allies the UK, Canada, New Zealand and Spain opposed their use.
The vast majority sent are artillery shells with a lower than 2.35% "dud rate", a reference to the percentage of bomblets which do not explode immediately and can remain a threat for years.
The weapons are effective when used against troops in trenches and fortified positions, as they render large areas too dangerous to move around in until cleared.
Russia has used similar cluster bombs in Ukraine since it launched its full-scale invasion last year, including in civilian areas.
Reacting to the US decision to send the bombs, Russian President Vladimir Putin said his country had similar weapons and they would be used "if they are used against us".
Oleksandr Syrskyi, the Ukrainian general in charge of operations in the country's east, told the BBC last week that his forces needed the weapons to "inflict maximum damage on enemy infantry".
"We'd like to get very fast results, but in reality it's practically impossible. The more infantry who die here, the more their relatives back in Russia will ask their government 'why?'"
He added however that cluster bombs would not "solve all our problems".
He also acknowledged that their use was controversial, but added: "If the Russians didn't use them, perhaps conscience would not allow us to do it too."